This application is based on French Patent Application No. 01 00 139 filed Jan. 5, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates generally to radio transmitter devices, in particular to mobile telephones, and more particularly to microstrip antennas included in such devices.
2. Description of the Prior Art
A microstrip antenna includes a patch that is typically obtained by etching a metal layer. This kind of antenna is known as a microstrip patch antenna.
The microstrip technique is a planar technique that has applications in producing lines and antennas providing coupling between lines transmitting signals and radiated waves. It uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate. A conductive layer on the bottom surface of the substrate constitutes a ground of the line and the antenna. The patch is typically wider than the strip and its shape and dimensions constitute important characteristics of the antenna. The shape of the substrates is typically that of a rectangular plane sheet of constant thickness, and the patch is also typically rectangular. However, varying the thickness of the substrate can widen the pass-band of the antenna and its patch can be various shapes, for example circular. The electric field lines between the strip or the patch and the ground layer pass through the substrate.
Antennas constructed in accordance with these techniques typically, although not necessarily, constitute resonant structures adapted to support standing waves providing a coupling with waves radiated into space.
Various types of resonant structure can be produced using the microstrip technique and can support various resonant modes, which for succinctness are referred to hereinafter as xe2x80x9cresonancesxe2x80x9d. Broadly speaking, each resonance can be described as consisting of a standing wave formed by the superposition of two travelling waves propagating in two opposite directions along the same path, these two waves resulting from the alternating reflection of the same travelling electromagnetic wave at the two ends of that path. Using this mode of description, the latter wave propagates in an electromagnetic line consisting of the ground, the substrate and the patch and which defines a linear path of zero width. In fact this kind of wave has wave surfaces that extend transversely over the whole of the section that is offered to them by the antenna, and thus this mode of description simplifies the real life situation, to a degree that is sometimes excessive. To the extent that it can be considered to be linear, the path can be rectilinear or curved. It will be referred to hereinafter as a xe2x80x9cresonance pathxe2x80x9d. The frequency of the resonance is inversely proportional to the time taken by the progressive wave referred to above to travel the length of that path.
A first type of resonance might be called xe2x80x9chalf-wavexe2x80x9d resonance. In this type of resonance the length of the resonance path is typically substantially equal to one half-wavelength, i.e. to half the wavelength of the travelling wave referred to above. The antenna is then referred to as a xe2x80x9chalf-wavexe2x80x9d antenna. This type of resonance can be generally defined by the presence of an electrical current node at each of the two ends of the path, whose length can therefore be equal to said half-wavelength multiplied by an integer other than 1. That integer is typically an odd number. Coupling with radiated waves is obtained at one end of the path at least, the ends of the path being situated in regions in which the electric field in the substrate has a maximum amplitude.
A second type of resonance that can be obtained using the same technique might be referred to as a xe2x80x9cquarter-wavexe2x80x9d resonance. It differs from a half-wave resonance, firstly, in that the resonance path typically has a length substantially equal to one quarter-wavelength, i.e. one quarter of the wavelength defined above. To this end the resonant structure must include a short circuit at one end of the path, the term xe2x80x9cshort circuitxe2x80x9d referring to a connection between the patch and ground. Also, the short circuit must have an impedance that is sufficiently low to impose such resonance. This type of resonance can be generally defined by the presence of an electrical field node fixed by this kind of short circuit in the vicinity of an edge of the patch and by an electrical current node situated at the other end of the resonance path. The length of the resonance path can therefore also be equal to said quarter-wavelength plus an integer number of half-wavelengths. Coupling with the waves radiated into space is obtained at an edge of the patch in a region in which the electric field through the substrate has a sufficiently large amplitude.
Resonances of other, more or less complex, types can be obtained in antennas of this kind, each resonance being characterized by a distribution of the electric and magnetic fields that oscillate in an region of space including the antenna and its immediate vicinity. They depend in particular on the configuration of the patches, which can in particular incorporate slots, possibly radiating slots. They also depend on the presence and location of any short circuits and on electrical models representing the short circuits if they are imperfect, i.e. if they cannot be regarded, even approximately, as perfect short circuits of zero impedance.
The present invention finds an application in diverse types of devices, such as mobile telephones, base stations for mobile telephones, automobile vehicles, aircraft and missiles. In the case of a mobile telephone, the continuous nature of the bottom ground layer of a microstrip antenna limits the radiation that is intercepted by the body of the user of the device when it is transmitting. In the case of automotive vehicles, and above all in the case of aircraft or missiles whose external surface is made of metal and has a curved profile to achieve low aerodynamic drag, the antenna can be conformed to the profile so as not to cause any troublesome additional aerodynamic drag.
The present invention relates more particularly to the situation in which a microstrip antenna must have the following qualities:
it must be a dual frequency antenna, i.e. it must be able to transmit and/or receive efficiently radiated waves on two frequencies separated by a large spectral gap, p1 it must be possible to connect it to a signal processor unit by means of a single connecting line for all operating frequencies of a transmitter device without giving rise to a troublesome spurious standing wave ratio on that line, and
it must not be necessary to use a frequency multiplexer or demultiplexer to achieve this result.
Many prior art microstrip antennas that have the above three qualities have been produced or proposed. They differ in terms of the means employed to obtain a plurality of resonant frequencies. Three such antennas will be examined:
A first prior art antenna of the above kind is described in U.S. Pat. No. 4,766,440 (Gegan). The patch 10 of this antenna is generally rectangular in shape and the antenna has two half-wave resonances with resonance paths along a length and a width of the patch. It also includes a U-shaped curved slot which is entirely inside the patch. The slot is a radiating slot and produces a supplementary resonance along another resonance path. By appropriately choosing its shape and its dimensions, the slot produces required values of the frequencies of the resonances, which provides the facility to transmit a circularly polarized wave by associating two modes having the same frequency and crossed linear polarizations with a relative phase of 90xc2x0. The coupling device takes the form of a microstrip line which is also coplanar in that the microstrip is in the plane of the patch and penetrates between two notches of the patch. The device includes impedance converter means for matching it to the various input impedances respectively presented by the line at the various resonant frequencies used as operating frequencies.
This first prior art antenna has the following drawbacks, among others:
The necessity to provide impedance converter means complicates its production.
It is difficult to adjust the resonant frequencies accurately to required values.
A second prior art antenna is described in U.S. Pat. No. 4,692,769 (Gegan). In a first embodiment the patch of this antenna is in the form of a circular disk 10 and the antenna has two half-wave resonances. The coupling system takes the form of a line 16 constituting a quarter-wave transformer and connected to a point inside the area of the patch so as to impart substantially equal values to the real part of the input impedance of the antenna for the two resonances. The line 16 is a microstrip line. Two slots are formed in the conductive layer of the patch and penetrate into the area thereof from its periphery to delimit between them the strip of a terminal segment of the line. One of the two slots is continued by an extension that constitutes an impedance matching slot 28.
This second prior art antenna has the following drawbacks, among others:
It is difficult to produce the impedance converter means.
It is difficult to adjust the resonant frequencies accurately to required values.
A third prior art dual frequency antenna differs from the previous ones in that it uses a quarter-wave resonance. It is described in the following paper: IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM DIGEST, NEWPORT BEACH, Jun. 18-23, 1995, pages 2124-2127 Boag et al xe2x80x9cDual Band Cavity-Backed Quarter-wave Patch Antennaxe2x80x9d. A first resonant frequency is defined by the dimensions and the characteristics of the substrate and the patch of the antenna. A matching system produces a resonance of substantially the same type at a second frequency on the same resonance path.
This third prior art antenna has the following drawbacks, among others:
The difference between the two resonant frequencies is too small in some applications.
The necessity to use a matching system complicates the production of the antenna.
The necessity to use a matching system complicates the production of the coupling device of the antenna in the form of a coaxial line.
The present invention has the following objects, among others:
a dual frequency antenna that is simple to manufacture,
a freer choice than previously of the ratio of the center frequencies of the two operating bands of a transmitter device, and more particularly an antenna for the device such that the ratio of the two usable resonant frequencies of the antenna is from approximately 1.25:1 to approximately 5:1 and in particular around 2:1,
a pass-band of the antenna that is sufficiently wide around each of these two resonant frequencies for one transmit frequency and one receive frequency of the device to be situated in each of the two bands,
easy and accurate adjustment of the two resonant frequencies,
use of a single coupling device for each of the two resonant frequencies, the impedance of which is easily adaptable, and
limited antenna dimensions.
With the above objects in view, the present invention provides a planar antenna including superposed layers respectively constituting:
a conductive ground,
a dielectric substrate formed on the ground, and
a patch formed on the substrate,
wherein the patch has an area and a periphery and includes a separator slot having an origin on the periphery and a closed end in the area, the closed end leaves a passage between itself and the periphery, the slot penetrates into the area from the origin and cooperates with the periphery to delimit in the area a first region and a second region, the two regions are conductive and electrically separated from each other by the slot and connected by the passage, the regions have respective areas, and the antenna further includes a reactive component mutually coupling the two conductive regions.
The reactive component is preferably flat, for example a surface mount component, which means that there is no significant projection from the planar structure of the antenna. For example, it is a capacitor having an area less than the area of each of the first and second regions, the area is less than the area of the patch and extends continuously over the first region, over the separator slot at a distance from the closed end and over the second region, and the capacitor is formed by the superposed layers cooperating with the patch and respectively constituting:
a dielectric layer formed on the patch, and
a conductive armature formed on the dielectric layer.
A flat reactive component can nevertheless have a different shape to provide coupling in accordance with the present invention. For example, it can be an interdigitated capacitor integrated into the trace of the separator slot by appropriate cut-outs in the facing edges of the two regions of the patch.
The antenna preferably further includes a short circuit electrically connecting the first conductive region to the ground in the vicinity of the origin of the separator slot.
The area of the capacitor is preferably from 1% to 25% of the area of the patch.
Preferably, the origin of the separator slot is close to the short circuit so that the two resonances have respective resonance paths which both extend from the short circuit, one of the two paths extending only in the first region and the other one extending in the first and second regions.
Various aspects of the present invention will be better understood from the following description and the accompanying diagrammatic drawings. When components are shown in more than one figure of the drawings, they are designated therein by the same reference numbers and/or letters.